Pierre Bergé, 86, died in his sleep at his home in Saint-Rémy-de-Provence on Friday after a long illness – myopathy, his foundation said in a statement.

The French businessman and cultural philanthropist was for decades a dynamic driving force in French fashion and the arts and leaves a significant footprint behind in each, somewhat intertwined.

Early on, he dreamed of being a writer and was a close friend of Jean Cocteau and Jean Giono but instead became a fashion executive.

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For decades, he was the diminutive but formidable presence at the side of his long-time life and business partner, Yves Saint Laurent. Bergé and Saint Laurent met in 1958, shortly after the latter was appointed head designer of the House of Dior, and from then on, their domestic and professional lives were linked. Saint Laurent and Bergé formed their own haute couture house in 1961, and he later steered the company into contemporary relevance amid the burgeoning French prêt-à-porter business with the launch of lucrative Saint Laurent Rive Gauche in 1966 and later, brand extensions into perfume and beauty. Bergé protected the frail creative genius in life and business, and after Saint Laurent's death in 2008, devoted himself to protecting their creative heritage.

In 2004, Bergé established the Fondation Pierre Bergé-Yves Saint Laurent. Its mission, through conservation of its extensive, 20,000-item design archives and thousands of sketches and materials, is to safeguard that heritage, organize exhibitions around the world and support educational and cultural works. In his later years, Bergé devoted himself to that legacy as well as to the arts (theatre and especially the Paris Opera, where he had been appointed president by friend and former French president François Mitterrand).

Under his auspices the Fondation also supported several literary prizes, including the Prix Jean Giono. Eventually, he did fulfill his original wish to be a man of letters himself, as the writer of a number of books, essays and monographs on Saint Laurent. The elegiac Lettres à Yves, a slim, poetic volume of rhetorical letters written after Saint Laurent's death, is a tender and moving glimpse into Bergé's profound joy and loss.

In 2009, crowds lined up for days to see Christie's auction of Yves Saint Laurent and Pierre Bergé's personal effects and lifetime of collected furniture and art. Their sumptuous rue de Babylone apartment was disassembled and recreated at the Grand Palais, and the 733 lots netted nearly half a billion dollars. In the ensuing documentary, L'Amour Fou, he narrates the profoundly sad billet-doux and elliptical look at their life and work together.

Bergé life's work could be said to be Yves Saint Laurent – the man, the company, the artist, the legacy. Of this, he was a vigilant and vocal guardian. When a competing pair of dramatic biopics came out in 2014, for example, the Fondation backed the more reverential version whereas Kering, the luxury conglomerate that now owns the current incarnation of the fashion brand that Yves built, supported the edgier and less-flattering, peak-hedonism interpretation. But he was also outspoken on a number of political issues, from LGBT rights in France (of which he was a fierce advocate) to what he saw as vulgar incursions on fashion's hallowed ground. "They are going to kill couture," he famously said of the 1990s presentations after LVMH hired designers John Galliano and Alexander McQueen, who brought hype and spectacle to the traditional French couture runway.

Sadly, Bergé died on the eve of the Fondation opening two museums that are arguably the culmination of his life's work. One in Marrakesh, a milieu of Saint Laurent inspiration since 1966, and the Musée Yves Saint Laurent Paris, occupying the historical hôtel particulier at 5 avenue Marceau, which was the couturier's long-time studio and headquarters. Both will open in October and should cement Pierre Bergé as a national treasure alongside his beloved Saint Laurent.

Editor’s Note: An earlier version of this article incorrectly said the former president of France was Frédéric Mitterrand when in fact, his name was François Mitterrand. This version has been corrected.